Wall-flow honeycomb filters are commonly used to remove solid particulates from fluids, such as in exhaust gas streams. FIG. 1 illustrates a typical honeycomb filter 100 with an inlet end face 102 for receiving the inlet gas stream, an outlet end face 104 for expelling the outlet gas stream, and an array of generally parallel intersecting porous walls 106 extending longitudinally from the inlet end face 102 to the outlet end face 104. The intersecting porous walls 106 define a honeycomb network of channels 107 including a plurality of inlet cell channels 108 and a plurality of outlet cell channels 110. The outlet cell channels 110 are closed with plugs 112 where they adjoin the inlet end face 102 and open where they adjoin the outlet end face 104. Oppositely, the inlet cell channels 108 are closed with porous plugs (not shown) where they adjoin the outlet end face 104 and open where they adjoin the inlet end face 102. Honeycomb filters 100 are typically secured in a compliant mat and contained in a rigid housing (not shown). Fluid directed at the inlet end face 102 of the honeycomb filter 100 enters the inlet cell channels 108, flows through the interconnecting porous walls 106 and into the outlet cell channels 110, and exits the honeycomb filter 100 at the outlet end face 104.
In a typical cell structure, each inlet channel 108 is bordered on one or more sides by outlet cells 110, and vice versa. The inlet and outlet channels 108, 110 may have a square cross-section as shown in FIG. 1 or may have other cell geometry, e.g., circular, rectangle, triangle, hexagon, octagon, etc. Diesel particulate filters are typically made of ceramic materials, such as cordierite, aluminum titanate, mullite or silicon carbide. When particulates, such as soot found in exhaust gas, flow through the interconnecting porous walls 106 of the honeycomb filter 100, a portion of the particulates in the fluid flow stream is retained by the interconnecting porous walls 106. The efficiency of the honeycomb filter 100 is related to the effectiveness of the interconnecting porous walls 106 in filtering the particulates from the fluid. Filtration efficiencies in excess of 80% by weight of the particulates may be achieved with honeycomb filters. However, filtration efficiency or integrity of a honeycomb filter can be compromised by various defects, such as holes or cracks (such as fissures) and the like in the walls or plugs. Such defects allow the fluid to pass through the filter without proper filtration. Thus, in the manufacture of honeycomb filters, it may be desirable to test the honeycomb filters for the presence of such defects that may affect filtration efficiency or integrity. Honeycombs with detected defects may be repaired, or if irreparable, discarded.